The US air force and NASA have launched a joint research push to advance hypersonic flight technology. The air force research lab and space agency are seeking university and industry partners, and are offering $30m in funding.
Hypersonic for this purpose is defined as five times the speed of sound or faster. Thus far, very few vehicles have operated in such flight regimes. In general, when seeking to move extremely fast around the world, it is more normal to lift a craft out of the atmosphere on a rocket - as in the case of intercontinental missiles. A few jet aircraft can briefly exceed Mach 3 (the MiG-25 Foxbat interceptor could do so just once if required, knackering its engines in the process), and the famous turbo-ramjet SR-71 "Blackbird" Cold War spyplane could reach such speeds repeatedly. But so far no known runway-landing, air-breathing plane has beaten Mach 4.
More: http://www.theregister.co.uk/2008/09/23/nasa_usaf_hypersonics_funding/